In third generation (3G) wireless communication systems, particularly in the time division duplex (TDD) mode, the time axis is divided into intervals of equal durations called frames. Cellular systems employing the TDD mode divide frames into a finite number (NT) of intervals of equal duration, called slots, and allow a cell to use some or all of the slots for uplink (mobile-to-base) or downlink (base-to-mobile) transmissions.
The slot assignment of a cell defines how each slot is used by the cell. It is generally understood that there are three possible ways for a cell to use a slot: (1) for uplink transmissions, (2) for downlink transmissions, or (3) not used.
The assignment of slots within a cell can be varied by the system to conform to the long-term variations of offered traffic. For example, the system may modify the assignment of one slot from uplink to downlink if the intensity of offered downlink traffic increases while the offered uplink traffic decreases. In addition, different cells of a system do not generally need to have the same slot assignment. Thus, if offered traffic characteristics in one geographical area are different from another geographical area, the cells covering those areas may have different assignments so as to best adapt to local traffic conditions.
While the capability of varying the assignment of slots within a cell as described in the previous paragraph permits the system to adapt to varying traffic asymmetry conditions by trading downlink capacity for uplink capacity (or vice versa) within a cell, it does not permit the system to adapt very well to geographically inhomogeneous traffic conditions (i.e., it will not allow a cell to simultaneously support more traffic in both directions than its neighbors). This is because a slot in prior art arrangements is not allowed to be used for both uplink and downlink transmissions at the same time. Therefore, the system will fail to handle the following scenarios: (1) where many users are geographically concentrated in the same area; or (2) where there are several users in the same area, requiring high bit rates in both uplink and downlink directions.
To some extent, such scenarios can be handled by a load balancing mechanism whereby some users do not connect to the most favorable cell in terms of path loss, but rather to neighboring cells that are less loaded. However, this approach has limitations in that it requires higher transmission power for users connected to distant cells. This can be a significant problem particularly for high data rate users (i.e., scenario (2) above). As a result, some users in heavily loaded cells cannot be served even though there is spare capacity elsewhere in the system. This is a disadvantage.
It would be desirable to provide a method of increasing the uplink and downlink cell capacity without the disadvantages of the known arrangements.